Have you tried geonames? I've not specifically used it for the US but that kind of information is in there for other countries. Only drawback of using geonames is that it can be a bit of effort getting what you want out of their feed.

This seems to suggest the data you're after is in there: http://www.geonames.org/postal-codes/postal-codes-us.html

Yep - with the possible exception of exact ZIP boundaries, it should all be on the download server: http://download.geonames.org/export/dump/

Grab the US.zip file and the English-language feature codes, filter down the data to the stuff you're interested in, and you're good to go.

For $40 these guys have something that also includes area code, time zone and is recent data & quick download, if that matters for your project:

http://zipcodedownload.com/Products/Product/Z5Commercial/Sta...

Regions for states is not something well defined, at least that I've found, as the definition varies depending on purpose (commerce, agriculture, political, ...). If you find something good, I'd love to hear about it.

for region to states, I found this - http://www.infoplease.com/ipa/A0770177.html

Let me know if that is what you were looking for

Hi hhimanshu - I totally mis-understood what you meant - if you were looking to group states into national regions, your link looks pretty good. I was thinking of regions in terms of grouping counties into logical/meaningful state regions. I do that thus far for CA and NY where in each I have about 10 regions.

I'm not entirely sure what you're trying to accomplish, but the USPS TIGER database can (sometimes) be useful in this area: http://zip4.usps.com/ncsc/addressmgmt/tiger.htm

Here's a direct link to a CSV of the 2000 Census Fips Region/Zip code, which I believe is taken from the USPS TIGER DB:

http://www.census.gov/tiger/tms/gazetteer/zips.txt

All I need is a mapping from county to cities, which is missing in this data too

This is awesome, it has loads of data, I can use it all, thank you

this doesn't seem to have any link that gives this result

If you aren't looking for an API solution, you might want to check this out: http://zips.sourceforge.net/

It isn't the most recent data, but it ought to get you started.

CivicSpace released a CSV db of that a while back. http://www.boutell.com/zipcodes/

look up melissa data at http://www.melissadata.com/products/zip-data.htm

Have you tried looking at factual.com?